Heat-enduring ferrous alloy casting



Jan. lo, 1939. B'. J. SAYLES 2,143,090

HEAT ENDURING FERROUS ALLOY CASTING Filed Nov. 12, 1939 2 sheets-sheet 1 Cf `.25 Z

/45 Cast Conc/dzon d./YJ D Fly 2.

Cf 2.5 Z

Hamed www af 1500 u:

INVENTOR erzram J ,Sayles Jan. l0, 1939. B. J. SAYLES 2,143,090

HEAT ENDURING FERROUS ALLOY CASTING Filed Nov. l2, 1938 2 SheetsSheet 2 Patented Jan. 10, 1939 UNITED STATES PATENT OFFICE This invention relates to heat enduring ferrous alloy castings and more particularly to such castings containing carbon,molybdenum, chromium y and nickel and also other elements such as silicon and manganese all Within particular rangs which impart very desirable properties to the castings.

The present application is a continuation-iny part of my application Serial No. 148,507, led June 16, 193'?.

In the accompanying drawings, the four figures are reproductions o. microphotographs of various ferrous alloy castings containing about 25% chromium and 10% nickel.

Fig. 1 shows the microstructure in the as cast condition of an alloy containing .60% carbon;

Fig. 2 illustrates the microstructure of the alloy shown in' Fig. 1 after heating the alloy for 24 hours at 1800 F.; l 20 Fig. 3 illustrates the microstructure of an alloy containing .35% carbon after the alloy has been heated for 24 hours at 1800 F.; and y Fig. 4 illustrates the microstructure of an alloy made in accordance with the present invention 25 and containing .35% carbon and 1.0% molybdenum, the alloy having been heated for 1,000 hours at 1800 F.

Heat enduring chromium nickel ferrous alloys are widely employed as castings required to sus- 80 tain high loads at high temperatures. Typical applications of such castings are found in tube supports used in oil still furnaces, walking beams, roller rails, 'hearth rollers, hearth supports, conveyor chains, itorts and other parts used in the 85 construction and operation of continuous mechanized furnaces used for heating or heat treating metals and metal products.

One of the most widely used alloys for industrial furnace construction and for products 40 used in heating is an alloy containing about 35% nickel and chromium, balance iron. -By reason of its high nickel content, this alloypresents the drawback of being expensive. Likewise, it is not particularly satisfactory where the 45 fuel used in. the furnace contains substantial quantities of sulphur, since nickel-chromium al-V loys'in which nickel is the predominating alloy element are subject to destructive corrosion in sulphur bearing furnace atmospheres.

In some industrial furnace applications, and almost invariably in the manufacture of -tube supports for oil stills, wide use has been made of alloys containing about to 30% chromium and 8% to 15% nickel. 5l in this group are substantially cheaper than the Chromium-nickel alloys high nickel compositions. A typical alloy in this groupv containing about chromium and 10% nickel-is equal in oxidation resistance to the more expensive 35% nickel-15% chromium composition where ordinary furnace atmospheresare involved. y Both materials are commercially useful up tomaxmum metal temperatures of about 2000 F. In furnaces employing high sulphur fuel, whichwould invariably include most oil'stills, the 25% chrome-10% nickel alloy exhibits marked superiority to the high nickel material by reason of its' greater immunity to corrosion in contact with sulphur furnace atmospheres.

At high temperatures, metals do not exhibit 1 a definite yield point under load stress so that the designing engineer is unable to select a safe Working load at high temperature by consideration of the yield point, as is customary in the design of metal structures intended to operate at room temperature. When samples of heat enduring alloys are maintained under load stress at high temperaturefor extended periods, it is found that a load very much less than that which represents the apparent "yield point in a quick time test is suiiicient to rupture the specimen if the load test be continued for a suicient length of time. There has thusl arisen the practice of conducting long time creep tests in which the observer notes the amount of load per square inch required to produce stretch or creep in the specimen at a definite rate such as 1% in 10,000 hours or 1% in 100,000 hours. The safe working load is takenras -a percentage of ,these creep values. For example, the safe working stress may be 50% of the 1%-1 0,000v hour creep value where the part in service will operate satisfactorily after a relativelyk large Vamount of distortion or may` be quickly and cheaply replaced. In other cases, 50% to '75% of the `1%-100,000 hourl creep value may be usedas the safe working load Whereslight distortion renders the part unserviceable or its replacement in event of failure is diiiicult or expensive.

It has long beendesired to produce chromiumnickel ferrous alloys having high `strength at high temperatures and good ductility at room temperature after the casting has been subjected to high temperatures for an extended period; Both of these 'qualities arev required, if a heatfenylur-v ing .casting is to satisfactorily" meet the requirements of its various uses. Heat enduring ferrous alloy castings containing about 25% chromium, about 10% nickel, and aboutf.60% carbon have been found to possess excellent high temperature Elongation in 2" (percent) load carrying capacity. Such an alloy at 1800 F. will sustain a load of 2,000 pounds per square inch while creeping at the rate of 1% in 10,000 hours. Unfortunately, a casting of this composition is unstable at highvtemperatures, as shown by the fact that after it has been heated to high temperatures for an extended period of time and then tested at room temperature it has very low ductility. Micro-examination of the material' indicates this loss of ductility to be due to migration of carbides to the grain boundaries.

It will be seen from Figs. 1 and 2 that in both the as cast condition and after heating for 24 hours at 1800 F. the iron and chromium carbides have migrated Although there is considerable migration ofthe carbides to the grain boundaries even in the as cast condition, this migration is increased by the heating. In Fig. 2, the migration of carbides has been such as to form a massive continuous network at the grain boundaries. They form a contnuous cellular structure in which the ferrite grains are practically completely surrounded by a massive continuous network.- These carbides are strong but brittle. They impart high creep values to the casting but when the alloy is tested for ductility at room temperature after having been heated at high temperature for an extended period, it is found that its ductility is very low indeed. When stress is applied to the metal, slippage of the grains results in fracture of the brittle grain boundary carbides and consequent rupture of the specimen after only a small amount of elongation. This is clearly shown by the following test:

EXAMPLE I Chromium 25.0% nickel 10.0%-carbra 0.60% As casttested at room temperature Tensile strength (lbs. per sq. in.) 77,000 11.0

After heating 1000 hours at 1800 F.-tested at room temperature Tensile strength (lbs. per sq. in.) 72,575 Elongation-in 2" (per cent) 2.0

creep vaine at 1800 F. 1%1o.oo hours) (lbs. per sq. in.) 2000 It will be noted that when the alloy is heated for 1000 hours at 1800 F. and tested at room temperature, the elongation in 2" is reduced from 11.0% to 2.0%.

It has been attempted to increase the ductility of chromium-nickel ferrous alloy castings by lowering the carbon content from the .60% carbon referred to. Thus alloys containing about 25% chromium, nickel and .35% carbon have been made. These have proved unsatisfactory because, although they have sufficient ductility when tested at room temperature after extended heating at high temperature, they do not have sufficient high temperature represents such a casting after heating for 24 hours at 1800 F. VHere again, as in the alloy containing .60% carbon (Fig. 2), the carbides have migrated to the grain boundaries and occur in rather massive particles. However, there is insufficient carbon present to form a complete network along the grain boundaries.

and heating at high temperature for periods d oes not materially reduce this ductility, since the carbon is inluiiicient to form to the grain boundaries.

'for an extended period.

' ductility at room strength. Fig. 3`

room temperature Tensile strength ..-(lbs. per sq. in.) 87,025 Elongation in 2" (per cent) 24.3

Creep value at 1800 F. (1%-10,000 hours) (lbs. per sq. in.) 1175 It will be noted, however, that improvement in high temperature stability thus obtained by re- -duction in carbon content is at the sacrifice of high temperature load carrying capacity. The lower carbon alloy of Example II has a 1%-10,000 hour creep value at 1800 F. of only 1175 lbs. per square inch, compared to a corresponding value of 2,000 lbs. per square inch for the higher carbon material of Example I. This would necessitate employment of almost twice the metal section to sustain the load.

'I'he use of chromium-nickel ferrous alloys containing about 23-28% chromium and about 8-15% nickel has been greatly restricted in its commercial application, due to the fact that it has been impossible heretofore to obtain both the desired high creep value and the desired high ductility when tested at room temperature after the alloy has been heated to a high temperature In highly mechanized industrial'heating furnace, the unexpected breakage of any part of the mechanism results in shutdown of the furnace and auxiliary equipment dependent upon its operation. In oil stills, tubes are removed and replaced by new ones when the still is cold and the alloy is in its most brittle condition. Frequently these tubes are warped or swollen so that substantial force is needed for their removal. Tube supports, being designed to carry vertical load, are lacking in transverse strength and, thus, are not particularly adapted to resist the thrust imposed by tube removals. Breakage of a tube support presents the problem tolf an expensive replacement and prolonged shutown.

I have found that I can produce a heat enduring ferrous alloy casting having high load carrying ability at high temperature, high stability at high temperature and a high degree of temperature following a period of heating at high temperature, provided that the various elements of the alloy hereinafter more particularly referred to are maintained within certain rather definite ranges. Substantial deviation from these ranges results in alloys which do not have all of the desired properties. The chromium in my alloy is between about 23 and 28% and preferably between about 25 and 28%. The nickel may-be as low as about 8% or as high as about It is preferred that it should be between about 9 and 12%. The molybdenum is maintained between about .75% and 1.75% and in the preferred alloy is between about .75% and 1.25%. 'I'he carbon may be about .25% to about .45% but preferably is between .30 and .40%. If the carbon is lower than about .25%, the alloy will not have sumcient strength or creep value at high temperature. On the other hand, if the carbon is above about .45%, the carbides migrate and precipitate at the grain boundaries during u After heating' 1000 hours at 1800 llitested at heating, forming a massive continuous network which is brittle and which imparts very low ductility to the casting when tested at room temperature. S

The molybdenum employed in my alloy retards the migration of carbides to the grain boundaries. Its effect is illustrated in Fig. 4. It will be noted that the carbides are relatively small in size and are scattered throughout the grains and grain boundaries, so as to internally reinforce the grains and form a fine discontinuous network at the grain boundaries. The iine carbide particles scattered throughout the structure within the grain boundaries reinforce the ferrite grains, giving them strength so that the alloy has high creep values at elevated temperatures. The network of carbides at the grain boundaries is fine and discontinuous, as contrasted with the massive continuous network at the grain boundaries as shown in Fig. 2. The structure of Fig. 4 may be described as consisting of ferrite grains in ternally reinforced by small sized carbides within the grains, the ferrite grains making discontinuous ferrite to ferritecontactwith each other at the grain boundaries. 'I'hus the internal reinforcement of the ferrite grains imparts high strength at high temperatures and the discontinuity of the carbide network at the grain boundaries prevents fracture when stress is applied, thereby resulting in good elongation.

If the molybdenum is below aboutv .75%, it does not have suicient retarding effect on the carbides and they migrate to the grain boundaries. The

internal grain reinforcement by carbides is lacking and the alloy does not have high creep value. On the other hand, if stantially above about 1.75%, the desired form, distribution and amount of carbide formation is not obtained and the alloy does not exhibit high creep value. It is thus necessary, in order to produce the desired internal grain structure and carbide network at the grain boundaries to maintain the elements, particularly the carbon and molybdenum, within the ranges which I have specied.

The silicon in my alloy should be between .25%

and 1.25% and is preferably between about .5% and 1.0%. vI have found that silicon in amounts materially higher than specified radically reduces .the ductility of the alloy when tested at room temperature after heating at elevated temperature. The manganese in my alloy is maintained between .25% and 1.50% preferably between about .4% and .9%. The balance of the alloy is substantially all iron. A preferred specific alloy according to my invention is:

The physical properties of my alloy are shown by the following example:

. EXAMPLE IWA i Chromium 25.0%-m'ckel' 10.0%-carb0n 0.35%

. molybdenum 1,0%

As cast-tested at room temperature Tensile strength (lbs. per sq. in.) 90,900 Elongation in 2" (per cent) 27.0

' Elongation in 2" (per cent) the' molybdenum is sub- A room temperature Tensile strength (lbs. per sq. in.) 84,275 18.5

creep value at 1soo F. (1ct-'10,000 hours) (lbs. per sq. in.) 1650 Although it is preferred that the molybdenum be about 1.0%, good results are obtained -with a molybdenum content of 1.72%, as shown by the following example:

EXAMPLE V Chromium 25.0%-m'ckel 10.0%-carhon 0.35%-

molybdenum 1.72%

As cast-tested at room temperature It will be observed that with the higher molybdenum content (Example V) the elongation as cast is not as high as with the lower molybdenum content (Example IV), but the loss of ductility after heating is less. Although the heating period in Example V was only 24 hours as compared to the 1000 hours heating period of Example IV, it has been established by tests that the heating period of 24 hours is suiiicient to detect instability inan alloy of this type.

Although in some alloys tungsten may be employed in place of molybdenum `and still obtain the desirable characteristics, I have found that such is' not the case in my alloy and that tungsten when used in place of molybdenum does not produce an alloy having the advantageous propertiesj which have been described.

A heat enduring ferrous alloy casting according to my invention and containing about .25%

to .45% carbon, about .75 to 1.75% molybdenum, about 23 to 28% chromium, about 8 to 15% nickel, about .25 to 1.50% manganese, and about .25 to 1.25% silicon, the balance being substantially all iron is characterized in the as cast condition by a creep rate of not more than 1% in 10,000 hours under a load of 1650 lbs. per square inch at 1800 F. on a creep test of not less than 1,000 hours duration. 'It hasan elongation 'in 2 of at least 9% when heated for 24 hours at 1400 F. and furnace cooled to 400? F. and tested at room temperature. It has a tensile strength of atleast 80,000 pounds per square inch and a yield point of at least 35,000 pounds per square inch when tested at room temperature after having been heated for 24 hours at 1400 F. and furnace cooled to 400 F. In obtaining these values the tensile strength is determined at a speed of crosshead not to exceed 1" per minute. The yield point is determined as that load which produces an extension of .006" in a 2" gauge length of the specimen using a free crosshead speed not to exceed .03" per minute. It is to be noted that in this type of alloy there is no sudden drop of the crosshead, which ordinarily would indicate the yield point. p

The heat enduring ferrous alloy castings o1' the present invention are particularly useful as oil' still tube supports but may be used for other purposes, such as roller hearths, chain conveyors, walking beams, roller rails or hearth supports for heat treating furnaces, or for carburizing trays,

boxes and retorts both for solid compound and gaseous atmosphere carburization.

The invention is not limited to the particular examples which have been given for purposes of illustration but may be otherwise embodied within thev scope of the following claims.

I claim:

1. A heat enduring ferrous alloy casting containing about .25 to .45% carbon, about .75 to 1.75% molybdenum, about 23 to 28% chromium, about 8 to 15% nickel, about .25 to 1.50% manganese, and about .25 to 1.25% silicon, the balance being substantially all iron, and characterized in the as cast condition by a creep rate of not more than 1% in 10,000 hours under a load of 1650 pounds per square inch at 1800 F. on a creep test of not less than 1,000 hours duration, the specimen when heated for 24 hours at 1400 F. and furnace cooled to 400 F., when tested at room temperature having an elongation in 2 of at least 9%, a tensile strength of at least 80,000 pounds per square inch and a yield point of at least 35,000 pounds per square inch.

2. A heat enduring ferrous alloy casting containing about .30 to .40% carbon, about .'75 to 1.25% molybdenum, about 25 to 28% chromium, about 9 to 12% nickel, about .4 to .9% nese, and labout .5 to 1.0% being substantially all iron, and characterized in the as cast'conditionby a creep rate of not more than 1% in 10,000 hours under a load of 1650 pounds per squareinch at 1800 F. on a creep test of not less than 1,000 hours duration, the specimen when heated for 24 hours at 1400 F. and furnace cooled to 400 room temperature having an elongation in 2" of at least 9%, a. tensile strength of at least 80,000 pounds per square inch and a yield point of at least 35,000 pounds per square inch.

3. A heat enduring ferrous alloy casting conmanga: silicon, the balancev F., when tested at taining about .25 to .45% carbon, about .75 to 1.75% molybdenum, about 23 to 28% chromium, about 8 to A15% nickel, about .25 to 1.50% manganese, and about .25 to 1.25% silicon, the balance being substantially all iro'n, and characterized in the as cast condition by a creep rate of not more than 1% in 10,000 hours under a load of 1650 pounds per square inch at 1800 F. on a creep test of not less than 1,000 hours duration, the specimen when heated for 24 hours at 1400 F. and furnace cooled to 400 when tested at room temperature having an elongation in 2 of at least 9%, a tensile strength of at least 80,000 pounds per square inch and a yield point o1' at least 35,000 pounds per square inch, and further characterized in that the carbides are scattered throughout the grain and grain boundaries internally reinforcing the grains and forming a ne discontinuous network at the grain boundaries.

4. A heat enduring ferrous alloy casting containing, about .35% carbon, about 1.0% molybdenum, about 25.0% chromium, about 10.0% nickel, about .60% manganese, and about .70% silicon, the balance being substantially all iron, and

characterized in the as cast condition by a. creep rate oi' not more than 1% in 10,000 hours under a load of 1650 pounds per square inch at 1800 F. on a creep test of not less than 1,000 hours duration, thespecimen when heated for 24 hours at 1400 F. and furnace cooled to 400 F., when tested at room temperature having an elongation in 2 of at least 9%, a tensile strength o! at least 80,000 pounds per square inch and a yield point of at least 35,000 pounds per square inch, and further characterized in that the carbides are scattered throughout the grain and grain boundaries internally reinforcing the ,grains and forming a ne discontinuous network at the grain BERTRAM J. SAYLES.

boundaries. 

